A haversack, used in the way it is supposed to does not need any waterproofing, as its a food bag designed to carry greasy salt pork, not your camera, car keys and wallet......
in the European military tradition they were quickly sewn up and used for a campaign season and then discarded. During the Revolution very few American soldiers happened to have both a knapsack and a haversack, carrying their food in their pockets or some other way.
Here is JJ Henry's account of what they call a "Pork Bag" and how they used it when they needed to repair their bark canoe. Henry, of Course was in one of the PA Rifle companies headed to Quebec with Morgan.
"A lucky thought occurred to the youngest of the company, that the pork bag, lay empty and neglected, in one of the canoes. The thought and the act of bringing it were instantaneous. The bag was ripped, and as if it had been so much gold dust, we scraped from it about a pint of dirty fat."